The present invention relates to a bipolar transistor. Furthermore, the invention relates to a method for fabricating a bipolar transistor.
Bipolar transistors are generally constructed from two pn junctions lying close together in a semiconductor crystal. In this case, either two n-doped regions are isolated from one another by a p-doped region (so-called npn transistors) or two p-doped regions are isolated from one another by an n-doped region (pnp transistors). The three differently doped regions are designated as emitter (E), base (B) and collector (C). Bipolar transistors have already been known for a long time and are used in manifold ways. A distinction is made between so-called individual transistors, which are intended for mounting on printed circuit boards or the like and are accommodated in their own housing, and so-called integrated transistors, which are fabricated together with further semiconductor components on a common semiconductor carrier, generally designated as a substrate.
The maximum oscillation frequency fmax of a bipolar transistor is proportional to the root of fT divided by 8πRBCBC, where RB is the base resistance, CBC is the base-collector capacitance and fT is the transition frequency. In order to obtain high oscillation frequencies, it is desirable, therefore, to reduce the base resistance. The base resistance of a bipolar transistor is determined both by the resistance of the connection region and by the sheet resistance of the base doping profile. This sheet resistance, the so-called pinch resistance, is inversely proportional to the base thickness given homogenous base doping. However, an increase in the base thickness leads to lengthening of the base transit time for the minority carriers.
An increase in the homogeneous base doping above 5×1018 reduces the breakdown voltage of the emitter-base junction to excessively low values and simultaneously increases the capacitance of the base-emitter depletion layer. A known method for further reducing the base pinch resistance is the use of a lightly doped (1×1018), epitaxial emitter. The light emitter doping allows the base to be doped practically up to 1×1020 without the blocking capability of the emitter-base junction being lost. Owing to the increased base charge compared with the emitter, the current gain of such a transistor would be too low, but this can be compensated for by the use of germanium in the base.
Previous concepts for producing structures with an epitaxial emitter are illustrated and described for example in Behammer et al., Solid State Electronics Vol. 41, No. 8, pp. 1105–1110 (1997) or J. Schiz et al. IEEE (1997), ISBN 7803-4135-X, pp. 255–260.
FIG. 1 diagrammatically shows a bipolar transistor in accordance with the publication by Behammer et al. in a simplified manner. The bipolar transistor first of all has a collector 102, which is formed in a silicon substrate or in a silicon epitaxial layer. The SiGe base 104 (p-doped) is provided on the collector 102 (n-doped) and the n type emitter 106 is provided on the base 104. A p+-type implantation 108 is arranged in a manner laterally adjoining the collector 102, the base 104 and the n-type emitter 106, the implantation producing the contact to the p-doped base. For the connection of the p+-type implantation 108, a metal layer 110 is provided, which is insulated from an n+-type emitter 114 by a so-called sidewall spacer 112. The n+-type emitter 114 in turn is arranged above the n-type emitter 106. The entire bipolar transistor is insulated from further components by an insulation 116 and an insulation layer 118.
In accordance with the publication by Behammer et al., the bipolar transistor shown in FIG. 1 can be fabricated with the aid of the so-called “blanket epitaxy”. In this method, however, dry etching is affected on the base connection region.
FIG. 2 diagrammatically shows a bipolar transistor in accordance with the publication by J. Schiz et al. in a simplified manner. The bipolar transistor 100 likewise has a collector 102 which is formed in a silicon substrate or in a silicon epitaxial layer. The SiGe base 104 (p-doped) is provided on the collector 102 (n-doped) and the n−-type emitter 106 is provided on the base 104. A p+-doped polysilicon layer 120 is arranged in a manner laterally adjoining the base 104 and the n−-type emitter 106, the polysilicon layer producing the contact to the p-doped base. An n+-type emitter 114 bounded by an aligned sidewall spacer 122 is arranged above the n−-type emitter 106. The entire bipolar transistor is again insulated from further components by an insulation 116 and an insulation layer 118.
The bipolar transistor in accordance with the publication by J. Schiz et al. has to produce the sidewall spacer 122 by means of a phototechnology, with the result that a thickness of less than 200 μm is generally not possible. However, such a thick sidewall spacer 122 results in significantly increased parasitic capacitances.
A common feature shared by the bipolar transistors shown in FIGS. 1 and 2 is that a so-called “link implantation” (contact implantation) is carried out outside the emitter region in order to reduce the base connection resistance. In a further publication by Harame et al., Trans. ED Vol. 42, No. 3, pp. 469–482, FIG. 3 therein illustrates the implantation damage, point defects, which are generally produced in the case of such implantation, and also in the case of dry etching on the silicon substrate.
Even assuming that the point defects are not extended into the active base region, the point defects present nonetheless lead to an abnormally high diffusion of the dopant boron into the nearby SiGe base 104. In order to prevent such dopant diffusion, the sidewall spacer cannot be made arbitrarily thin. In order to keep the point defects away from the base, a sidewall spacer of approximately 150 nm or larger is necessary, which, however, increases the link resistance and the base collector capacitance. For the case where the sidewall spacer is completely omitted and the implantation is additionally effected into the connection region, a functioning component cannot be expected. Furthermore, the base profile is greatly widened during subsequent temperature steps above 550 C on account of the point defects still present.